I miss placed my solution to calibrate my pH meter right before I was to brew my mead. I looked up some common liquids with pH and found that Milk has a constant 6.7 to 6.8 pH. So I calibrated my pH meter with milk I let sit to room temps. I rinse my meter after every test and I never put the solution back into the carboy.
After the brew session and I was putting everything away I found my calibration solution... for giggles I tested it out, and sure enough my pH meter was calibrated to the 2nd decimal (7.00 that's a tenth right, man I'm getting old)
I think I'm going to stick to milk instead of buying calibration solutions. Since milk manufactures are articulate about pH of their milk.